He was a good character in the sense of his more intimate understanding of the reason what/who Gantz is , what he was capable of , and what he was there for. The entire manga irked me a bit with the such naive understanding that some of the major characters had. But it was their personalities that helped build the concept of humans' faith in themselves, each other, and their fortitude/will. Nichi was the complete asshole, he know it himself, but did it for a reason. Him spouting off about how Kei and the others were idiots, and ridiculed them with social darwinism drives home the fact hes looking to be the most powerful (the ubermensch) amongst everyone else.

could he of redeemed himself? certainly. but would he of done it out of the love he has for the human race? lol , ask how he feels about his (former) classmates hahah.

The Gantz seemed to be able to revive the dead without restrictions in the last chapter. Personally, I like to think that he died and then got revived, because it feels as if he was still developing as a character.

What about letting Izumi be killed? That's some fucked up shit, he was just as good as Kei was in a fight, possibly better. I'd have preferred the characters he killed doing him in instead of some random fucking Vampires.